The present disclosure generally relates to stimulation of subterranean formations, and, more specifically, to methods and systems for stimulation of subterranean formations in the presence of a metal ion.
Treatment fluids can be used in a variety of subterranean treatment operations. Such treatment operations can include, without limitation, drilling operations, stimulation operations, production operations, remediation operations, sand control treatments, and the like. As used herein, the terms “treat,” “treatment,” “treating,” and grammatical equivalents thereof refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid or a component thereof, unless otherwise specified herein. More specific examples of illustrative treatment operations can include drilling operations, fracturing operations, gravel packing operations, acidizing operations, scale dissolution and removal operations, sand control operations, consolidation operations, and the like.
Acidizing operations may be used to stimulate a subterranean formation to increase production of a hydrocarbon resource therefrom. Introduction of the acidizing fluid to the subterranean formation may take place at matrix flow rates without fracturing of the formation matrix, or at higher injection rates and pressures to fracture the formation (i.e., an acid-fracturing operation). During an acidizing operation, an acid-soluble material in the subterranean formation can be dissolved by one or more acids to expand existing flow pathways in the subterranean formation, to create new flow pathways in the subterranean formation, and/or to remove acid-soluble precipitation damage (i.e., scale) in the subterranean formation. The acid-soluble material being dissolved by the acid(s) can be part of or formed from the native formation matrix or can have been deliberately introduced into the subterranean formation in conjunction with a stimulation operation or like treatment operation (e.g., proppant or gravel particulates). Illustrative substances within the native formation matrix that may be dissolved by an acid include, but are not limited to, carbonates, silicates and aluminosilicates.
Carbonate formations can contain minerals that comprise a carbonate anion and a metal counter ion (e.g., calcite (calcium carbonate) and dolomite (calcium magnesium carbonate)). When acidizing a carbonate formation, the acidity of the treatment fluid alone can be sufficient to solubilize the carbonate mineral by decomposing the carbonate anion to carbon dioxide and leeching a metal ion into the treatment fluid. As the concentration of dissolved metal ions rises, particularly at higher pH values upon spending of the acid, the solubility limit may be exceeded and precipitation of scale may occur. Both mineral acids (e.g., hydrochloric acid) and organic acids (e.g., acetic and formic acids) can be used to treat a carbonate formation, often with similar degrees of success.
Siliceous formations can include minerals such as, for example, zeolites, clays, and feldspars. As used herein, the term “siliceous” refers to a substance having the characteristics of silica, including silicates and/or aluminosilicates. Dissolution of siliceous materials through an acidizing operation is thought to be considerably different than acidizing carbonate materials, since the mineral and organic acids that can be effective for acidizing carbonate materials may have little effect on a siliceous materials. In contrast, hydrofluoric acid, another mineral acid, can react very readily with siliceous materials to promote their dissolution. Oftentimes, a mineral acid or an organic acid can be used in conjunction with hydrofluoric acid to maintain a low pH state as the hydrofluoric acid becomes spent during dissolution of a siliceous material. In addition to siliceous materials, many types of siliceous formations can also contain varying amounts of carbonate materials. Most sandstone formations, for example, contain about 40% to about 98% sand quartz particles (i.e., silica), bonded together by various amounts of cementing materials, which may be siliceous in nature (e.g., aluminosilicates or other silicates) or non-siliceous in nature (e.g., carbonates, such as calcite). Reprecipitation of scale can also occur when acidizing a siliceous formation due to secondary reactions of dissolved silicon species.
Calcium ions and other alkaline earth metal ions can be particularly problematic when acidizing both siliceous and non-siliceous subterranean formations. For either type of subterranean formation, the solubility limit of dissolved metal ions can be quickly exceeded and deposition of scale may occur. In the case of siliceous formations being acidized with hydrofluoric acid, dissolved calcium ions can react readily with free fluoride ions to generate highly insoluble calcium fluoride scale. Other metal ions may be similarly problematic in this regard. Calcium fluoride and other types of scale formed from metal ions can be highly damaging to subterranean formations, possibly even more so than if the initial acidizing operation had not been performed in the first place.
One approach that has been used to address the issues associated with dissolved metal ions is to employ chelating agents, which can sequester the metal ions in a more soluble and less reactive form of a metal-ligand complex. As used herein, the terms “complex,” “complexing,” “complexation” and other variants thereof refer to the formation of a metal-ligand bond without reference to the mode of bonding. Although complexation of a metal ion may involve a chelation process, complexation is not deemed to be limited in this manner. Once bound in a metal-ligand complex, a metal ion may be more soluble and have a significantly decreased propensity to undergo a further reaction to form damaging scale.
There are difficulties associated with chelation strategies, however. At low pH values, the carboxylic acid groups of many chelating agents may be substantially protonated, a form that can be ineffective for promoting metal ion complexation. This issue can significantly limit the working pH range over which an acidizing operation may take place, potentially limiting the acidizing operation's speed and effectiveness. Environmental concerns may also be problematic for some chelating agents. Although a wide variety of chelating agents are known, there may be a very limited number with a favorable environmental profile and availability in sufficient quantities to effectively support various types of subterranean treatment operations. The commercially available alkali metal form of many common chelating agents can also be problematic for acidizing operations when the alkali metal content becomes too high.